This invention concerns an optical recording disk, and to be more detailed, concerns an optical recording medium, with which the worsening of jitter can be prevented effectively even when fingerprints become adhered onto an incidence surface for laser light.
Optical recording disks, as represented by CDs and DVDs, have been used widely as recording media for recording digital data, and in recent years, optical recording disks of the next generation, which are greater in capacity and have higher data transferring rates, are being developed actively.
In developing a next-generation optical disk with greater capacity and higher data transferring rate, the beam spot diameter of a laser light used for the recording and reproduction of data must inevitably be made extremely small.
In order to make the beam spot diameter of a laser light small, the numerical aperture (NA) of an objective lens for converging the laser light must be made large and the wavelength λ of the laser light must be made short, and the use of an objective lens with a numerical aperture (NA) of 0.85 to converge a laser light with wavelength λ in the blue light range (wavelength of 380 to 450 nm) for recording data into a next-generation optical recording disk and reproducing data from the optical recording disk has been proposed.
However, when the numerical aperture (NA) of the objective lens for converging the laser light is made high, the tolerable angular error of the tilting of the optical axis of the laser light with respect to the optical recording medium, that is, the tilt margin T becomes extremely narrow. The tilt margin T decreases as the NA of the objective lens increases and increases as the thickness d of a light transmitting layer decreases. It is thus effective to make the thickness d of the light transmitting layer small in order to prevent the tilt margin T from becoming small.
Thus for a next-generation optical recording disk, the widening of the tilt margin by thinning of the light transmitting layer to approximately 100 μm has been proposed.
However, when the thickness of the light transmitting layer, through which laser light is transmitted, is thinned to approximately 100 μm, the area of the laser light beam spot on the top surface of the light transmitting layer becomes small. Error will thus tend to occur readily during the reading of data in cases where a laser light incidence surface of the light transmitting layer is flawed or has debris or fingerprints adhered thereon.
It has thus been proposed that a next-generation optical recording disk, which is arranged for illumination of laser light through a light transmitting layer and is premised on use in households, be used in a state in which it is housed in a cartridge.
However, that a next-generation optical recording disk cannot be used unless it is housed in a cartridge, despite CDs and DVDs being used in a so-called bare disk form in which the disk is not housed in a cartridge, is extremely inconvenient for users, and if it becomes essential for next-generation optical recording disks to be housed in cartridges, the next-generation optical recording disks will also become high in cost.
A next-generation optical recording disk, having a hard coat layer that is hard formed on the top surface of the light transmitting layer and can be used in the bare disk form, has thus been proposed.
Such an optical recording disk, having a hard coat layer that is hard formed on the top surface of the light transmitting layer, has the merit of not having to be housed in a cartridge and enabling use in the same manner as a CD or a DVD.
However on the other hand, since an optical recording disk, having a hard coat layer that is hard formed on the top surface of the light transmitting layer, is premised on use in the bare disk form, fingerprints tend to become adhered onto the surface of the hard coat layer, and when a fingerprint becomes adhered, the jitter during reading of data becomes large and error tends to occur readily.